Charge coupled devices (CCDs) (e.g., photodetectors, analog to digital converters, signal processors, etc.) are known. Such devices are used for accomplishing charge domain operations such as charge packet transfer, summation, and splitting. CCDs are useful in certain signal processing operations such as decoding of radio frequency signals. For example, charge domain filters have been constructed which provide finite impulse response (FIR) filtering or infinite impulse response filtering (IIR) filtering, often from within a single CCD device at very low rates of power consumption.
Charge domain analog to digital (A/D) converters are further examples of CCD devices that process signals in the charge domain. Charge domain A/D converters, in fact, are often used in conjunction with charge domain FIR or IIR filtering devices because signals received by a radio frequency receiver in the frequency domain are often more efficiently processed in the charge domain before conversion to the digital domain within the charge domain A/D converter.
One of the listed charge domain operations, charge splitting, is of critical importance in charge domain analog to digital (A/D) or digital analog (D/A) converters in that it is used to implement fractional multiplication by binary weighting (i.e., 1/2) and therefore is one of the fundamental operations in A/D and D/A converters. Charge splitting, as a consequence, determines conversion accuracy, and therefore the converter characteristics, such as linearity, spurious response and effective bits. Charge splitting serves a similar functionality in charge domain finite impulse response (FIR) and infinite impulse response (IIR) filters.
It has been suggested that charge splitting may be accomplished by creating a field oxide barrier in a charge transfer channel and clocking a charge packet into adjacent but isolated reservoirs separated by the barrier. Such a suggestion is based upon the assumption that charge distribution of the charge packet in the charge channel passing the barrier is uniform and reproducible. The suggestion is also based upon the assumption that the barrier can be accurately located within the charge channel and that the divided charges may then be later removed from the isolated reservoirs without recombining the previously divided charge.
Charge splitting error produced under the previously suggested methods have ranged from as high as a few percent to as little as a few tenths of a percent. Efforts aimed at improving accuracy have included increasing the width of the charge channel (to reduce the impact of an inaccurately placed charge splitter) and placing dummy splitters in the charge channel to compensate for nonlinearities caused by edge effects and to equalize lateral forces applied to charge carriers. Because of the importance of charge splitting in CCDs, a need exists for a method of improving the accuracy of charge splitting that is not dependent upon process parameters used in creating the CCDs.